Many electrical applications require current or power dissipation beyond the ratings of a single transistor. In these applications, two or more transistors, called pass transistors, are often connected in parallel to meet the current or power requirements of the circuit. Common applications in which pass transistors are used include power switching and power regulation.
To operate effectively, the pass transistors must share current equally to prevent problems of thermal runaway. Thermal runaway is a condition where one of a group of parallel-connected pass transistors begins to heat up, thereby lowering the base-emitter voltage drop (V.sub.BE) and increasing the gain (h.sub.fe) of the transistor. In turn, the fluctuation of V.sub.BE and h.sub.fe further increases the current flow through the transistor, creating additional heat that has a further effect on these parameters. Eventually, the current flow through the transistor may exceed the ratings of the transistor, causing failure. Merely connecting the pass transistors in a parallel configuration will not provide equal current sharing, because of the inherent characteristics of the transistors. In particular, the bipolar transistors used as pass transistors typically have a wide variation in the base-emitter voltage drops and different gain values.
A common method to ensure equal current sharing is to use current sense resistors, i.e., emitter resistors, to provide degenerative feedback, thereby masking the individual differences of the transistors. The voltage drop across the emitter resistors can also be monitored to prevent an over-current condition through the pass transistors. To work effectively, the resistors chosen should have a voltage drop at maximum current which is at least as large as the base-to-emitter voltage of the pass transistors, typically several tenths of a volt. The power lost in the current-sense resistors is typically not available for a useful purpose, and the power available to the load is therefore reduced. In addition, this extra power dissipation usually increases the complexity, cost, and size of the electrical system. More recently, operational amplifiers have been utilized to drive the pass transistors, allowing lower resistances to be used for the sense resistors.
The disadvantages associated with typical methods of providing current sharing become increasingly important when high efficiency is required at near-full load of the system, e.g., when the power being dissipated by the load approaches the limits of the total power available to the system. Continuing efforts are being made to provide current control and over-current protection without dissipating significant amounts of power for this purpose.